1. Field of the Invention
The present invention relates to control systems for use in supplying conditioned air to an enclosure. More particularly the present invention relates to a control system for coordinately synchronizing a heat pump and a furnace to condition the air of an enclosure.
2. Description of the Prior Art
It has been determined that a heat pump is capable of supplying sufficient quantities of heated air to meet many residential and commercial heating applications. The use of a heat pump to transfer heat from an area where loss of heat is not important to an area where heat is required is a very efficient method of heating an enclosure. Many heat pumps commercially available are capable of transferring up to two or three times the amount of heat from one area to another with a given amount of electricity as the amount of heat that would be generated if that same amount of electricity were used for electrical resistance heating. A heat pump having a high coefficient of performance may also be more efficient than a fuel fired furnace and with proper use results in an overall energy usage savings for a given amount of heating.
Heat pumps are, however, limited in overall application since for a heat pump to operate it must be capable of removing heat from one area and transferring that heat to the area or enclosure to be heated. Heat pumps of modern day design are capable of performing this operation at temperatures as low as 10 to 15 degrees Fahrenheit while maintaining a satisfactory coefficient of performance. However, below this temperature, the heat pump has to work so hard to absorb heat from the colder ambient air that its efficiency is decreased to the point where other forms of heating use less energy and consequently are less costly.
Heat pumps also have the disadvantage that when the refrigerant in the outdoor coil is being evaporated to absorb heat from the ambient air, the air adjacent thereto is cooled below the freezing point and as it is cooled the moisture in the air is precipitated onto the outdoor coil surface resulting in a frost or ice buildup thereon. As frost builds the coil rapidly loses its ability to transfer heat.
It has been found that below certain outdoor temperatures it is both economical and advantageous to use conventional furnace type heating to heat an enclosure. This may include the use of electrical resistance heat or a conventional gas or oil fired furnace. The point at which it is desirable to switch from the use of a heat pump to the use of the alternate heat source is called the balance point. This point may be chosen either based on the economics of operating the heat pump versus the furnace or may be chosen solely on the basis of the capability of the heat pump for supplying sufficient heat to the enclosure.
Typically many homes or residential applications have had a gas or oil burning furnace installed as original equipment when the residence was constructed. Many of these homes likewise have air conditioning equipment with a heat exchanger installed within the furnace supply duct to the area to be conditioned. For an application having a furnace and an air conditioner it is relatively simple to install a control such that the air conditioner is utilized as a heat pump when the weather conditions are such that the heat pump is more efficient and likewise to operate the furnace when it is more efficient. Furthermore it may be economical to install a heat pump in combination with an existing furnace to realize the economies of heating with the heat pump at relatively higher outdoor temperatures.
Considerable prior art exists in relation to the use of auxiliary heat sources with a heat pump. These auxiliary heat sources have typically been electrical resistance heaters incorporated within the heat pump unit such that no other heating source is required within the enclosure to provide adequate heating capacity at all normal outdoor temperatures.
Typical of the prior art utilizing electrical resistance heat in combination with the heat pump is U.S. Pat. No. 3,173,476 issued to McCready and assigned to Carrier Corporation, the assignee hereof. This patent discloses the use of an outdoor thermostat in series with a multi-stage indoor thermostat. The outdoor thermostat designated 58 therein is so arranged that the auxiliary electric heater may not be turned on until the outdoor temperature reaches a predetermined level. Once the outdoor temperature reaches that level then, when the indoor temperature falls more than a predetermined amount below the desired temperature, thermostat 47 is engaged and the auxiliary electrical heat is turned on. Prior to the auxiliary heating elements being energized the indoor thermostat determines when heating is needed and actuates the heat pump. The auxiliary heat is supplied in combination with the heat pump such that both operate to provide heating when the outdoor temperature is below a predetermined level and when the indoor temperature is a predetermined amount below the desired indoor temperature level.
Other prior art utilizing multi-stage indoor thermostats to determine when the auxiliary electric resistance heat should be commenced include Shell, Pat. No. 3,318,372; Ferdelman, U.S. Pat. No. 3,537,509; and Kyle, U.S. Pat. No. 3,556,203.
Other patents have utilized the temperature of the refrigerant returning to the compressor to ascertain when auxiliary heat is required. See U.S Pat. No. 3,283,809 issued to Eberhart and U.S. Pat. No. 3,404,729 issued to Armott.
Another method of controlling the transfer from heat pump heating to auxiliary heating is the utilization of a series of outdoor thermostats such that the auxiliary heat is brought on in stages by the various thermostats. For examples of this manner of control see U.S. Pat. No. 3,444,923 issued to Kyle, et al. and U.S. Pat. No. 2,806,674 issued to Biehn.
A further method of controlling the inter-reaction of auxiliary heat with the heat pump is disclosed in U.S. Pat. No. 2,902,220 issued to Myck, Jr., et al. Therein a step controller determines the length of time that indoor thermostats call for heat and based on the length of time activates auxiliary heaters in sequence.
A different method is shown in U.S. Pat. No. 3,537,509 issued to Ferdelman wherein upon the outdoor temperature reaching a certain level resistance heat is automatically engaged and heat pump operation discontinued and upon the indoor temperature continuing to fall thereafter auxiliary resistance heat is brought on line by a second stage of the indoor thermostat.
In U.S. Pat. No. 3,996,998 issued to Garst, et al., and entitled "Combination Furnace-Heat Pump Unit" there is disclosed a heat pump in combination with a furnace having various controls for determining whether the heat pump or the furnace will be operated. More particularly in the heating mode the thermostat 68 determines the temperature of the air within the enclosure and has a first threshold temperature at which the heat pump is activated. The heat pump alone supplies heat until a second lower indoor threshold temperature is reached. This lower temperature is reached because the heat pump is not able to meet the heating load under these particular weather conditions. At that time the furnace is activated for additional heating capacity and the heat pump operation is discontinued. A thermal-switch 56 is utilized such that the indoor coil of the heat pump is never allowed to be operated when the supply air temperature is contact therewith exceeds 115.degree. F. Consequently, whenever operation is switched from the furnace to the heat pump it is necessary that there be a delay involved to allow the supply air within the duct to cool. A temperature differential switch 60 is utilized such that when a predetermined differential as exceeded defrost is initiated. Consequently in defrost the heat pump is reversed and the furnace turned on such that the heat from the furnace may be used to help defrost the outdoor coil.
None of the above enumerated patents utilizes a combination heat pump and furnace wherein the control mechanism for selecting either heat pump operation or furnace operation is dependent solely on the outdoor ambient air temperature. In Eberhart, U.S. Pat. No. 3,283,809, a sensing element 44 is utilized to switch between heat pump operation and electrical resistance heat based on the lower of either the temperature of the ambient air or the refrigerant in the outdoor coil. In Ferdelman, U.S. Pat. No. 3,537,509, a temperature switch 143, being dependent on ambient air temperature, switches between first stage resistance heating and heat pump operation. Upon a further fall in indoor temperature the two stage room thermostat then switches on an additional auxiliary resistance heater. Furthermore, none of the references disclosed utilizes the defrost thermostat built into the heat pump for controlling the combination operation of the furnace and the heat pump. In particular, these references do not utilize the defrost thermostat to prevent excessive refrigerant pressures or temperatures in the heat pump system on changeover between furnace and heat pump operation when the unit is cycled in the defrost mode of operation.